1. Field of Invention
The invention relates to a phototransistor and, in particular, to a semiconductor phototransistor that uses a sidewall structure to increase the device speed and electrical bandwidth.
2. Related Art
Photodetectors (PDs) that use silicon crystals to replace group III-V compound semiconductors have the advantages of low costs, high yields, and high integratability in post circuits in the fiber communications market. However, their indirect bandgap, and relatively low electron mobility make the optoelectronic devices have worse performance than the group III-V semiconductors near the fiber communications wavelengths (xcx9c0.85 xcexcm, 1.3xcx9c1.55 xcexcm).
In the normal PDs made of silicon crystals, one needs to use the deep trench, silicon on insulator (SOI), and wafer bonding techniques in order for the speed to be greater than 2.5 GBits/sec. Such techniques not only greatly increase the costs, but also amplify the difficulty in integrating post circuit systems. In comparison, the heterojunction phototransistor (HPT) does not need to use the SOI substrate. Not only does it solve the substrate current problem, it is also relatively easy to integrate with the post bipolar transistor circuits.
Besides, the large photocurrent gain in such devices can compensate for the drawback that the silicon crystal has relative weaker photo absorption constant than III-V materials, achieving the required response. In comparison with the technique of another type of PDs with gains, the avalanche photodiode (APD), phototransistors not only are able to provide larger gains at lower operating voltages, they also do not require the use of complicated voltage control and temperature control circuits.
Traditionally, although the HPT""s have fairly high responses, the biggest problem is the speed performance. Therefore, the chances of their applications in digital optical fiber communications systems and their commercialization are limited.
There have been many phototransistor related techniques disclosed up to now. For example, the phototransistor disclosed in the U.S. Pat. No. 4,833,511 does not provide any solution in the speed but is only added with a quantum structure to improve the response. The phototransistor disclosed in the U.S. Pat. No. 5,844,253 adds a quantum dot structure to the emitter-base junction, using the thermal electron effect to improve the phototransistor speed performance. The phototransistor disclosed in the U.S. Pat. No. 6,525,348 adopts a conventional dual-end contact mode and uses an optimized base doping method to increase the speed. However, experimental results of the above-mentioned phototransistors are not as expected. Besides, the response speed performance of most phototransistors is around the ns level when operating under the condition that the photocurrent gain is not sacrificed.
When operating normal phototransistors, one usually adopts the base open mode. Its operation is to use photons to excite carriers, providing a virtual bias on the base-emitter (B-E) junction to obtain a large optical gain. However, the electrical bandwidth of phototransistors operating speed in such a mode can only reach the MHz level. It is mainly because the holes excited by the photons will accumulate at the B-E junction and cannot be removed. In order to solve the speed problem, the most direct method is to impose a bias voltage (VBE) or a bias current (IB at the B-E junction. However, this method has some drawbacks and becomes impractical. Such problems include huge dark currents (bias currents) and the resultant power consumption. The gain will also be attenuated because of the imposed bias voltage, therefore greatly increasing the dark current of the device and the standby power consumption.
It is therefore an important technical issue to improve the phototransistor speed performance without sacrificing the device gain and increasing device power consumption.
In view of the foregoing, an objective of the invention is to provide a phototransistor that has a better speed performance without sacrificing device gains and increasing the device power consumption.
An appropriately doped sidewall is added to the phototransistor structure. A sidewall contact is formed on the sidewall for directly removing the photon-excited holes accumulated at the base-emitter junction.
To achieve the above objective, the disclosed semiconductor phototransistor structure has a sidewall grown on the side of the collector and under the base by implant or epitaxy. A metal sidewall contact is then formed on the sidewall surface. A lowest voltage imposing on the metal contact is enough to remove the holes accumulated on the base-emitter junction, achieving the goal of improving the speed performance.
The disclosed phototransistor structure can decrease the hole removal time and thus speed up the phototransistor. On the other hand, the disclosed structure is completely compatible with the standardized SiGe bipolar transistor manufacturing process in the industry. Therefore, it has applications in opto-electric integrated circuits (OEIC""s).